1. Field of the Invention
The present invention relates to an optical device for dividing incident laser light into a plurality of light components and, more particularly, to an optical device suitably used as a crystallization device used to produce a crystallized semiconductor film by irradiating a non-singlecrystalline semiconductor film such as an amorphous silicon (a-Si) film with pulsed laser light having a predetermined light intensity distribution.
2. Description of the Related Art
A thin-film-transistor (TFT) used as a switching element for selecting display pixels of a liquid-crystal-display (LCD) is formed on an amorphous-silicon film or a poly-silicon film. The poly-silicon film is constituted of a large number of crystal grains, and hence when a TFT is formed on a poly-silicon film, a crystal grain boundary is formed in a channel region, and the crystal grain boundary becomes a barrier to the movement of electrons and positive holes. For this reason, there is a problem that in a poly-silicon film, mobility of electrons or positive holes becomes low as compared with the case of a singlecrystalline silicon film. Further, a number of TFTs formed on a poly-silicon film are different from each other in the number, position, and shape or the like of crystal grain boundaries formed in the channel region, which causes variations in the TFT characteristics, and causes, in the case of a liquid-crystal-display, a problem of display unevenness.
In order to improve mobility of electrons or positive holes, and reduce variations in the TFT characteristics, a phase control ELA (excimer laser annealing) method is proposed in Surface Science vol. 21, No. 5, pp. 278-287 (pp. 32-41), 2000 (Non-Pat. Document 1) and Jpn. Pat. Appln. KOKAI Publication No. 2000-306859 (Pat. Document 1) and the like. In the phase control ELA method, by using an illumination optical system provided with a homogenizer for homogenizing an in-plane light intensity distribution and a phase shifter for phase-modulating incident laser light to form a light intensity profile of strength and weakness, it is possible to form pulsed laser light having a desired light intensity profile of strength and weakness and form a crystallized region of a large grain size by irradiating a desired position of a non-singlecrystalline silicon thin film with the thus formed pulsed laser light. Specifically, the light receiving region of the non-singlecrystalline silicon thin film irradiated with the pulsed laser light having a desired light intensity profile of strength and weakness is melted, as a solid-liquid separation position moves in the lateral direction in accordance with the light intensity profile of strength and weakness in the process of a temperature fall of the melted region in the shutoff period the laser light, the crystallized position also moves in the lateral direction, and crystallized silicon constituted of crystal grains of having such a large grain size as to allow at least one channel region to be formed can be formed.
It is described in Jpn. Pat. Appln. KOKAI Publication No. 2005-311340 (Pat. Document 2) that in order to improve the degree of freedom of design of the optical system in the phase control ELA method, a particular type of abaxial cylindrical lens is used as a homogenizer. The cylindrical lens described in Pat. Document 2 is constituted of a lens array in which lens segments M having the same size and shape (semicylindrical shape obtained by cutting a cylinder in halves in the axial direction, and having the same curvature of the light transmission surface) are aligned as shown in FIG. 7A.
However, there has been a problem that when the in-plane light intensity distribution is homogenized by using the conventional cylindrical lens 300 described in Pat. Document 2, and a crystallization process is executed by using the crystallization device described in Non-Pat. Document 1 or Pat. Document 1, the light intensity profile of strength and weakness having a sinusoidal shape shown in Non-Pat. Document 1, p. 284 (p. 38), FIG. 9(a) cannot be formed. The present inventors have eagerly studied about the cause-and-effect relationship. As a result, it was found that a ringing (transient oscillation) phenomenon in which the light intensity unstably fluctuated was caused in the periphery of the transmitted light of the homogenizer as shown in FIG. 8A. It was found that in the region of the non-singlecrystalline semiconductor irradiated with light in which the ringing phenomenon is caused, crystallization of a desired size does not progress, and consequently, the crystallized region in the ringing region becomes small and a crystallized region having a size corresponding to a channel region cannot be formed, thereby lowering the yield of manufacture.
As the means for removing light in which a ringing phenomenon is caused, it is conceivable that a mask may be provided in the light transmission path of the homogenizer, there is however a problem that laser light from the laser light source cannot be utilized effectively.